The invention relates to detection of modulated signals for use in radio communication. More particularly, the invention relates to multi-level quadrature amplitude modulation (“M-QAM”) detection in communication systems.
In a digital communication system, digital symbols representing information are transmitted between different nodes (e.g., base stations, mobile phones) to exchange information.
A layered model often referred to as the Open System Interconnection (“OSI”) model is often used to describe communication systems. The lowest layer in that model, where information streams consisting of bits are transmitted, is often referred to as the physical layer, and information is often transmitted on different physical channels. A physical channel provides services with a predefined quality, depending on the arrangement. In a simplified description, a physical channel includes the formatting of bits in a predefined format, coding, interleaving, modulation of a carrier, transmission over a medium, down-conversion, demodulation, de-interleaving, and forward error correction. In addition, there are many other functions that are required for proper operation, such as synchronization in both time and frequency and channel estimation. Pilot symbols are often transmitted among information symbols on the physical channels. These pilot symbols are then used in the receiver to obtain synchronization and channel estimates. The channel estimates describe how the transmitted symbols are affected by the channel (including the modulation, TX front-end, medium, RX front-end, and demodulator), and are used to reconstruct the signal in the receiver. Put another way, the channel estimates help determine the radio channel's response so it can be compensated at the receiver.
Physical channels can be of three types, dedicated channels, shared channels, and broadcast channels. Broadcast channels carry common information to all or a group of users, while shared channels are data channels that can be used by many users. Dedicated physical channels are used by only one user at a time.
A medium carries the electromagnetic or optical signal between the antennas of the different nodes. In radio communication systems the medium consists of “free-space” and the signal is electromagnetic waves propagating in this medium.
A base station most often transmits using multiple physical channels. In time division multiple access (“TDMA”) systems, physical channels from the same base station are separated using time (and frequency if multiple carriers are used). In frequency division multiple access (“FDMA”) systems, only frequency is used to separate different physical channels. In code division multiple access (“CDMA”) systems, codes are used to separate different users (and frequency if multiple carriers are used).
At the receiver, a received signal is processed to obtain a sequence or stream of digital samples, called here “received samples” or a “received sample stream,” and these samples may be represented as complex numbers. For example, the received signal may be filtered, amplified, and mixed down to baseband using in-phase and quadrature local oscillators, and after analog-to-digital (“A/D”) conversion and synchronization processing, a stream of complex received samples is obtained. Each sample in the complex sample stream r(n) may be represented as a sum of a real component and an imaginary component, that is, r(n)=I(n)+jQ(n), where I(n) represents the in-phase components of the samples, Q(n) represents the quadrature components of the samples, and n is a sample time index.
Newer third generation (“3G”) cellular communications systems predominantly employ wideband code division multiple access (“WCDMA”) technology. An extension to the WCDMA standard known as high speed downlink packet access (“HSDPA”) has recently been standardized within the Third Generation Partnership Project (“3GPP”) standardization organization. HSDPA introduces known technologies such as higher order modulation and incremental redundancy to the 3GPP universal mobile telecommunications system (“UMTS”) standard. The higher order modulation introduced in HSDPA is M-QAM, and more particularly 16-state quadrature amplitude modulation (“16-QAM”), which in effect doubles the number of bits that can be transferred per radio channel use over prior systems.
Generally speaking, 16-QAM is achieved by modulating two four-level pulse amplitude modulated (“PAM”) signals onto two respective orthogonal carriers (I and Q), providing 42=16 possible symbol representations. Accordingly, a 16-QAM symbol includes phase information based on the respective I or Q orthogonal carrier and amplitude information. Quadrature phase shift keying (“QPSK”) detection, by comparison, includes only phase information.
Receivers in WCDMA systems also rely on a reference signal, such as time-multiplexed pilot symbols or a code-multiplexed pilot channel, to calculate estimates of a radio channel's response. Typically, the channel gain and phase of the common pilot channel (“CPICH”) is estimated once per slot for this purpose. The CPICH is a QPSK-modulated channel that includes only the relevant phase information, and therefore only provides a phase reference. The 16-QAM data, which is transferred on the high speed physical downlink shared channel (“HS-PDSCH”), requires processing of phase and amplitude information to recover the information in the data. The gain offset between the CPICH and HS-PDSCH, which is unknown to the receiver, must therefore be determined in order to establish an amplitude reference in addition to the phase reference for channel estimation and symbol detection.
Without an estimate of the gain offset, the decision boundaries for the corresponding 16-QAM constellation cannot be established for proper detection. An example of a normalized 16-QAM constellation having average power Ēs=2 is shown in FIG. 1. The constellation includes 16 equi-probable constellation points 100–115 and a grid of decision boundaries 120–125. Since the radio channel changes the gain and phase of the original signal over time, the decision boundaries 120–125 must be updated continuously, which requires information to be stored until the decision boundaries 120–125 can be estimated. Consequently, as the speed at which the CPICH/HS-PDSCH gain difference is estimated increases, the memory requirements in the receiver are decreased. It is therefore advantageous to estimate the gain difference as fast as possible to realize this savings in overhead. The gain difference estimator should therefore be able to estimate the gain difference fast, preferably within about half a slot.
Some methods have been disclosed for estimation of decision boundaries in M-QAM. For example, in “A Method for Blind Determination of Pilot to Data Power Ratio for QAM Signals,” TSG-RAN Working Group 1 #21, Aug. 27–31, 2001, estimation of the decision boundaries is based on an estimation of the ratio between the pilot channel's power and the data channel's power. Disadvantages of this approach include the fact that both a filtering and a division between two estimated values are required. The method is therefore quite complex and results in the production of noisy decision boundary estimates.
Other estimation methods require the estimation of the decision boundaries to be based on signal power estimation followed by a non-linear transform of the estimate. Disadvantages of this approach also include complexity and noisy estimates resulting from the non-linear transformations.
The problem with previous approaches is the required complexity of calculation, which results in slower estimation speeds, which results in increased overhead in terms of memory, and noisy decision boundary estimates. A need therefore exists for faster, simpler, and less noise prone M-QAM detection in communication systems.